The study of mammalian organogenesis by mosaic pattern analysis

Comparison of two related lines of tauGFP transgenic mice designed for lineage tracing

Background

The tauGFP reporter fusion protein is produced nearly ubiquitously by the TgTP6.3 transgene in TP6.3 mice and its localisation to microtubules offers some advantages over soluble GFP as a lineage marker. However, TgTP6.3^Tg/Tg homozygotes are not viable and TgTP6.3^Tg/− hemizygotes are smaller than wild-type. TP6.4 mice carry the TgTP6.4 transgene, which was produced with the same construct used to generate TgTP6.3, so we investigated whether TgTP6.4 had any advantages over TgTP6.3.

Results

Although TgTP6.4^Tg/Tg homozygotes died before weaning, TgTP6.4^Tg/− hemizygotes were viable and fertile and only males were significantly lighter than wild-type. The TgTP6.4 transgene produced the tauGFP fusion protein by the 2-cell stage and it was widely expressed in adults but tauGFP fluorescence was weak or absent in several tissues, including some neural tissues. The TgTP6.4 transgene expression pattern changed over several years of breeding and mosaic transgene expression became increasingly common in all expressing tissues. This mosaicism was used to visualise clonal lineages in the adrenal cortex of TgTP6.4^Tg/− hemizygotes and these were qualitatively and quantitatively comparable to lineages reported previously for other mosaic transgenic mice, X-inactivation mosaics and chimaeras. Mosaicism occurred less frequently in TP6.3 than TP6.4 mice and was only observed in the corneal epithelium and adrenal cortex.

Conclusions

Mosaic expression makes the TgTP6.4 transgene unsuitable for use as a conventional cell lineage marker but such mosaicism provides a useful system for visualising clonal lineages that arise during development or maintenance of adult tissues. Differences in the occurrence of mosaicism between related transgenic lines, such as that described for lines TP6.3 and TP6.4, might provide a useful system for investigating the mechanism of transgene silencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0149-x) contains supplementary material, which is available to authorized users.

Lessons from mouse chimaera experiments with a reiterated transgene marker: revised marker criteria and a review of chimaera markers

Recent reports of a new generation of ubiquitous transgenic chimaera markers prompted us to consider the criteria used to evaluate new chimaera markers and develop more objective assessment methods. To investigate this experimentally we used several series of fetal and adult chimaeras, carrying an older, multi-copy transgenic marker. We used two additional independent markers and objective, quantitative criteria for cell selection and cell mixing to investigate quantitative and spatial aspects of developmental neutrality. We also suggest how the quantitative analysis we used could be simplified for future use with other markers. As a result, we recommend a five-step procedure for investigators to evaluate new chimaera markers based partly on criteria proposed previously but with a greater emphasis on examining the developmental neutrality of prospective new markers. These five steps comprise (1) review of published information, (2) evaluation of marker detection, (3) genetic crosses to check for effects on viability and growth, (4) comparisons of chimaeras with and without the marker and (5) analysis of chimaeras with both cell populations labelled. Finally, we review a number of different chimaera markers and evaluate them using the extended set of criteria. These comparisons indicate that, although the new generation of ubiquitous fluorescent markers are the best of those currently available and fulfil most of the criteria required of a chimaera marker, further work is required to determine whether they are developmentally neutral.

Mechanics and spiral formation in the rat cornea

During the maturation of some mammals such as mice and rats, corneal epithelial cells tend to develop into patterns such as spirals over time. A better understanding of these patterns can help to understand how the organ develops and may give insight into some of the diseases affecting corneal development. In this paper, a framework for explaining the development of the epithelial cells forming spiral patterns due to the effect of tensile and shear strains is proposed. Using chimeric animals, made by combining embryonic cells from genetically distinguishable strains, we can observe the development of patterns in the cornea. Aggregates of cell progeny from one strain or the other called patches form as organs and tissue develop. The boundaries of these patches are fitted with logarithmic spirals on confocal images of adult rat corneas. To compare with observed patterns, we develop a three-dimensional large strain finite element model for the rat cornea under intraocular pressure to examine the strain distribution on the cornea surface. The model includes the effects of oriented and dispersed fibrils families throughout the cornea and a nearly incompressible matrix. Tracing the directions of critical strain vectors on the cornea surface leads to spiral-like curves that are compared to the observed logarithmic spirals. Good agreement between the observed and numerical curves supports the proposed assumption that shear and tensile strains facilitate sliding of epithelial cells to develop spiral patterns.

Cell proliferation, movement and differentiation during maintenance of the adult mouse adrenal cortex

Appropriate maintenance and regeneration of adult endocrine organs is important in both normal physiology and disease. We investigated cell proliferation, movement and differentiation in the adult mouse adrenal cortex, using different 5-bromo-2'-deoxyuridine (BrdU) labelling regimens and immunostaining for phenotypic steroidogenic cell markers. Pulse-labelling showed that cell division was largely confined to the outer cortex, with most cells moving inwards towards the medulla at around 13-20 µm per day, though a distinct labelled cell population remained in the outer 10% of the cortex. Pulse-chase-labelling coupled with phenotypic immunostaining showed that, unlike cells in the inner cortex, most BrdU-positive outer cortical cells did not express steroidogenic markers, while co-staining for BrdU and Ki67 revealed that some outer cortical BrdU-positive cells were induced to proliferate following acute adrenocorticotropic hormone (ACTH) treatment. Extended pulse-chase-labelling identified cells in the outer cortex which retained BrdU label for up to 18-23 weeks. Together, these observations are consistent with the location of both slow-cycling stem/progenitor and transiently amplifying cell populations in the outer cortex. Understanding the relationships between these distinct adrenocortical cell populations will be crucial to clarify mechanisms underpinning adrenocortical maintenance and long-term adaptation to pathophysiological states.

Three dimensional visualization and fractal analysis of mosaic patches in rat chimeras: cell assortment in liver, adrenal cortex and cornea

The production of organ parenchyma in a rapid and reproducible manner is critical to normal development. In chimeras produced by the combination of genetically distinguishable tissues, mosaic patterns of cells derived from the combined genotypes can be visualized. These patterns comprise patches of contiguously similar genotypes and are different in different organs but similar in a given organ from individual to individual. Thus, the processes that produce the patterns are regulated and conserved. We have previously established that mosaic patches in multiple tissues are fractal, consistent with an iterative, recursive growth model with simple stereotypical division rules. Fractal dimensions of various tissues are consistent with algorithmic models in which changing a single variable (e.g. daughter cell placement after division) switches the mosaic pattern from islands to stripes of cells. Here we show that the spiral pattern previously observed in mouse cornea can also be visualized in rat chimeras. While it is generally held that the pattern is induced by stem cell division dynamics, there is an unexplained discrepancy in the speed of cellular migration and the emergence of the pattern. We demonstrate in chimeric rat corneas both island and striped patterns exist depending on the age of the animal. The patches that comprise the pattern are fractal, and the fractal dimension changes with the age of the animal and indicates the constraint in patch complexity as the spiral pattern emerges. The spiral patterns are consistent with a loxodrome. Such data are likely to be relevant to growth and cell division in organ systems and will help in understanding how organ parenchyma are generated and maintained from multipotent stem cell populations located in specific topographical locations within the organ. Ultimately, understanding algorithmic growth is likely to be essential in achieving organ regeneration in vivo or in vitro from stem cell populations.

Transition from organogenesis to stem cell maintenance in the mouse adrenal cortex

Mice showing mosaic expression of an appropriate marker gene that is activated during development provide simple tools for investigating cell lineages. We used the mosaic β-galactosidase staining patterns in adrenal cortices of 21OH/ LacZ transgenic mice to study both organogenesis and maintenance of the adult tissue. Randomly orientated mosaic patterns present in embryonic day 14.5 (E14.5) adrenals changed progressively during the perinatal period from discrete spots, via patches and radial arrays, to radial stripes, which first emerged between postnatal days 0 and 7 (P0 and P7). The mosaic radial stripe pattern was fully established by P21 and remained unchanged throughout the adult period (8-52 weeks). The mouse adrenal gland grew continuously between E14.5 and P21, including the period during which stripes emerge. Ki67-positive, proliferative cells in the adrenal cortex were mainly localized to the outer cell layers between E18.5 and P3. By P10, cell proliferation had increased, and the proliferative region had expanded but was still mainly confined to the outer cortex. Correlation of changes in mosaic patterns in 21OH/LacZ adrenal cortices with the locations of adrenocortical cell proliferation suggest that the radial stripes arise by edge-biased growth during the perinatal period, even if they are maintained by stem cells in adults. The stability of the adult stripe pattern suggests that stem cell function is unchanged between 8 and 52 weeks.

In search of adrenocortical stem and progenitor cells

Scientists have long hypothesized the existence of tissue-specific (somatic) stem cells and have searched for their location in different organs. The theory that adrenocortical organ homeostasis is maintained by undifferentiated stem or progenitor cells can be traced back nearly a century. Similar to other organ systems, it is widely believed that these rare cells of the adrenal cortex remain relatively undifferentiated and quiescent until needed to replenish the organ, at which time they undergo proliferation and terminal differentiation. Historical studies examining cell cycle activation by label retention assays and regenerative potential by organ transplantation experiments suggested that the adrenocortical progenitors reside in the outer periphery of the adrenal gland. Over the past decade, the Hammer laboratory, building on this hypothesis and these observations, has endeavored to understand the mechanisms of adrenocortical development and organ maintenance. In this review, we summarize the current knowledge of adrenal organogenesis. We present evidence for the existence and location of adrenocortical stem/progenitor cells and their potential contribution to adrenocortical carcinomas. Data described herein come primarily from studies conducted in the Hammer laboratory with incorporation of important related studies from other investigators. Together, the work provides a framework for the emerging somatic stem cell field as it relates to the adrenal gland.

Intermingling versus clonal coherence during skeletal muscle development: Mosaicism in eGFP/nLacZ-labeled mouse chimeras

To study the relative contributions of clonal coherence vs. myoblast intermingling to the formation of mammalian skeletal muscles, enhanced green fluorescent protein (eGFP) and nLacZ labels were used to analyze neonatal and adult mouse embryo aggregation chimeras. The eGFP marker allowed us to estimate absolute levels of the eGFP transgenic parental contributions as well as variances of levels within a chimeric individual; nLacZ served as a counter-label in neonatal chimeras. In mature chimeric muscle, free intracellular diffusion of eGFP led to an averaging of the eGFP level in individual myofibers. Chimerism, i.e., differing eGFP levels, was evident between corresponding right and left muscles, between adjacent back, body wall, limb (shank, foot), and extraocular muscles and among myofibers within these muscles. Inhomogeneities in muscles far from their somitic origin indicated unbalanced sampling from small founder pools. The results obtained with adult eGFP<-->0 chimeras were corroborated by neonatal chimeras with complementary eGFP<-->nLacZ labels.

Mosaic analysis of small intestinal development using the spf(ash)-heterozygous female mouse

Mosaic analysis using the spf(ash)-heterozygous female mouse was performed to clarify the cell lineage and cell behavior during small intestinal development with special attention given to the villus and crypt formation. The spf(ash) mutation, located on the X-chromosome, causes ornithine transcarbamylase (OTC) deficiency, which leads to mosaic expression of this enzyme in the small intestine of the heterozygous female mouse. In the small intestine in heterozygous fetuses, very small patches, which were aggregates of OTC-positive cells or negative cells, with no definite orientation to the villus structures were observed. In the neonatal small intestine, the intervillus region (the presumptive crypts) was polyclonal, and the majority of crypts were comprised exclusively cells of either genotype in 2-week-old small intestine. These results suggest that extensive migration and cell mixing of small intestinal epithelial cells, which have no definite correlation with the villus formation, occur in fetal stages of development, and that the crypt morphogenesis commences after birth independently of the monoclonality of the epithelial cells. Our data with the mosaic mice also reconfirmed the monoclonality of the adult small intestinal crypts demonstrated in mouse aggregation chimeras.

Quantitative analysis of cell allocation during liver development, using the spf(ash)-heterozygous female mouse

Mosaicism of ornithine transcarbamylase (OTC) expression in hepatocytes was quantitatively analyzed during liver development of the spf(ash)-heterozygous female mouse. Because the mosaic patterns depend on cell migration and cell mixing, such analysis could give insights on the growth pattern or allocation pattern of hepatocytes during liver development. Complex mosaic patterns of OTC-positive and -negative hepatocytes were observed in sections of fetal and postnatal livers. Sizes of patches, which were aggregates of OTC-positive or -negative hepatocytes, increased during development. Patches were slender and comparatively simple in 15.5- and 17.5-day fetal and neonatal livers. Quantitative analysis of patch shapes demonstrated that undulation of patches was maximal at 7 postnatal days. Patches with nodular shapes also started to increase in number at this stage. Isolated patches in sections of fetal livers and postnatal livers three-dimensionally connected with one another. However, especially in fetal livers, in which OTC-positive patches were minor, due to the presence of abundant hemopoietic cells, isolated three-dimensional patches consisting of approximately 5 to 70 cells were often found. They were shaped like slender branching or zigzag-shaped cords, but no definite orientation such as portal-central was observed in them at any stage. These results suggest that hepatocytes contiguously allocate their daughter cells as zigzag-shaped or branching cords at younger stages. Some hepatocytes grow with nodular formation after 7 postnatal days. Migration and mixing of hepatocytes appear to be more extensive at fetal stages than in the adult liver. Immunohistochemical analysis of intercellular junction proteins (E-cadherin, connexins 26 and 32, occludin, and ZO-1) also revealed that their expression and distribution changed in hepatocytes during development, which may be correlated with the OTC mosaic patterns.

Fractal geometry of mosaic pattern demonstrates liver regeneration is a self-similar process

Partial hepatectomy causes compensatory, nonneoplastic growth and regeneration in mammalian liver. Compensatory liver growth can be used to examine aspects of patterns of cell division in regenerating tissue. Chimeric animals provide markers of cell lineage which are independent of growth and can be used to follow cell division patterns. Previous experimental evidence suggests that compensatory liver growth is uniform, without focal centers of proliferation. In this study we have extended that observation to include genes important in regeneration and cell cycle control in order to establish that nascent growth centers are not present in regenerating liver. There is a uniform spatial distribution of expression of these genes which is not related to mosaic pattern in the chimeras. While these genes may help regulate hepatocyte proliferation they do not appear to regulate patch pattern in the chimeras. With this information confirming uniform growth it was possible to use fractal analysis to test various hypothesized patterns of regenerative growth in the liver. The results of this analysis indicate that mosaic pattern does not change substantially during the regenerative process. Patch area and perimeter (the area occupied by or perimeter around cells of like lineage) increase during compensatory liver growth in chimeric rats without alteration of the geometric complexity of patch boundaries (boundaries around cells of like lineage). These tissue findings are consistent with previously reported computer models of growth in which repetitive application of simple decisions assuming uniform growth created complex mosaic patterns. They support the notion that an iterating (repeating), self-similar (a pattern in which parts are representative of, but not identical to the whole) cell division program is sufficient for the regeneration of liver tissue following partial hepatectomy. Iterating, self-similar cell division programs are important because they suggest a way in which complex patterns (or morphogenesis) can be efficiently created from a small amount of stored information.

Comparison of epidermal patch size in X-chromosome-linked mosaic and dizygotic chimeric mice

Mosaic animals can be made by aggregating embryonic tissues of distinguishable strains or they will occur spontaneously in eutherian mammals as a result of X-chromosome inactivation. Tissues of mosaic animals comprise aggregates of cells of similar lineage called 'patches'. The patch size of isolated epidermis from chimeras and X-chromosome-linked mosaics was compared in a quantitative fashion. Patch size was determined in the isolated epidermis of skin from aggregation chimeras between BALB/c and C3H/He strains of mice variant at the Gpi-1 locus and from the skin of X-chromosome-linked mosaic female BALB/c x C3H/He a mice heterozygous at the Pgk-1 locus. Patch size in this isolated tissue was not significantly different in these two types of mosaic animals. The results suggest that mechanisms in patch formation are primarily mechanical, dependent on cell division patterns.

Probabilistic division systems modeling the generation of mosaic fields

The explanation of mosaic pattern in chimeric organs analyzed by in situ methods requires modeling of specific hypotheses. The use of computer simulations to achieve this has led to the conclusion that finely variegated mixtures of cell lineage within chimeric tissues does not require extensive cell movement. Cell division models were used to determine the distribution of patch size as mosaic fields are generated. The results establish that these distributions are sensitive to the proportion of the two cell types which comprise the mosaic.

An in situ-hybridization study of the localization of retinol-binding protein and transthyretin messenger RNAs during fetal development in the rat

The patterns, and the cellular sites, of expression of the genes for retinol-binding protein (RBP) and transthyretin (TTR) were studied during early- to mid-stages of rat embryogenesis. In situ hybridization of single-stranded RNA probes to rat embryo sections revealed specific sites at which RBP messenger RNA (mRNA) and TTR mRNA were localized in rat conceptuses from 7 to 13 days of gestation. RBP and TTR mRNAs were both observed in the visceral endoderm as early as at 7 days of gestation. The transcripts were not expressed in the parietal endoderm. At 9 days of gestation, TTR mRNA was strongly detected in the visceral extraembryonic endoderm and in the foregut endoderm, whereas RBP mRNA was detected only in the visceral extraembryonic endoderm. From the 10th day to the 13th day of gestation, both transcripts were increasingly expressed in the visceral yolk sac endoderm and in the developing fetal liver, and gradually decreased in the foregut. At 11 days of gestation, TTR mRNA was first detected in the tela choroidea of the fourth ventricle in the brain; and at 13 days, the TTR mRNA was strongly evident in the developing choroid plexus of the fourth and lateral ventricles. These in situ hybridization studies with embryos at different developmental stages show that RBP and TTR mRNAs are transcribed quite early during embryogenesis. The protein products of these transcripts may play important roles in vitamin A and thyroid hormone metabolism, and in the functions that these regulatory compounds play, in the developing rat embryo.

Maintenance of foreign gene expression and independent chromosome fragments in adult transgenic rainbow trout and their offspring

Transgenic rainbow trout (RT) were generated in earlier experiments by an in vivo modification of the chromosome-mediated gene transfer technique. This involved fertilizing albino RT eggs with gamma-irradiated brook (speckled) trout (ST) sperm and then heat shocking the eggs to induce second polar body retention. Striped pigment patterns were stable in six mosaic individuals between the ages of 7 and 14 months. We estimated the total ST DNA contribution in adult transgenic fish using DNA dot blot analyses. An average of 7% ST DNA was obtained in analyses of 11 transgenic individuals. Nine of 55 transgenic individuals examined expressed a ST-specific isozyme in adulthood; some were mosaic for foreign gene expression. Variable numbers of autonomous chromosome fragments were maintained in 10 adult transgenic individuals examined. Five transgenic RT were backcrossed to albino RT. Chromosome fragments were inherited by 29 of 31 backcross embryos examined. Additional active nucleolar organizer regions (NORs) were present on some fragments and may also have been integrated into some host chromosomes. This method may have numerous applications for basic and applied genetic research in fish.

Mosaic pattern and lineage analysis in chimeras

Mammalian chimeras have been used in a number of developmental studies over the years. A major limitation in these studies has been the lack of in situ procedures for establishing mosaic pattern in the tissues of these animals. Recently, a number of procedures have become available for the histochemical demonstration of mosaicism in chimeras. These include the elucidation of various enzymes, receptors, or surface antigens, which have variant expression between strains. The observation of pattern in organs of mosaic animals can suggest possible modes of organogenesis and organ maintenance. Experimentation with such animals can be used to establish some mechanisms of pathogenesis as well.

Quantitative evaluation of coat-color patterns in artificially produced chimeras of the mouse by means of a microcomputer-based video-image analysis system

The possible application of microcomputer-based video-image analysis systems for the quantitative description of coat-color patterns in artificially produced chimeras and genetic mosaics of mice was investigated using a program developed by the author. This system is capable of extracting, from sampled images of pelts, the morphometric image features as defined by Pratt [1978] that are essential to the quantitative description of coat-color patterns in these animals. It does so with reasonable accuracy and speed and at low cost. No description of any similar system has been published in the literature. Performance of our system is described using C3H/HeJ----BALB/c chimeras as examples. The complex phenotypic expression of hair pigmentation in mice makes the use of a video-image analysis system like this one essential to evaluate the morphometric parameters of the patterns (e.g., the mixing ratios between the two components, the number of different-colored stripes, etc.) more precisely and reproducibly than has been done yet in the literature. The results indicate that the number of melanoblast clones in mice, as estimated from the number of minimal recognizable stripes (MRS), might be considerably larger than previously indicated; the figure presently obtained, i.e., 22.3 +/- 2.16 unilaterally in terms of the hypothetical maximum number of stripes (HMNS) (28.73 +/- 1.55, after correction for the random clumping) in the thoracicolumbar region of the mouse closely approximates the number of the somites in that region. Concerning the degree of mixing between the two components, it was proposed that the unmixed portion of the components derived from one strain increases in proportion to the second power of the increase in the relative total content of the same components. Work is in progress in our laboratory to analyze a large number of the chimeric pelts using the system described in this paper.